1. Field of the Invention
This invention relates to a method and apparatus for the direct, non-contact, real-time sampling and detection of minute quantities of materials on surfaces.
More particularly, this invention relates to a method and apparatus for producing ions from targeted sample molecules on or above a surface that is spaced apart from the apparatus and for detecting and identifying those ions, all without contacting the surface.
2. Description of Related Art
Military and security needs, law enforcement concerns and environmental monitoring all require a capability to sample and detect minute quantities of explosives, drugs, chemical and biological agents, toxic industrial chemicals, and other targeted compounds of interest residing on or in a variety of materials and surfaces. Most users desire a fast, portable, simple, operator friendly detector that combines different detection capabilities in a single unit and that is capable of directly and automatically acquiring samples from surfaces, identifying targeted substances in those samples, and providing immediate operator notification that such substances are present or not.
Most explosives, chemical warfare agent, toxic industrial chemical and illicit substance detectors in use for security purposes depend upon the vapor pressure of the targeted material for detection. If vapor pressures of chemicals of interest are very low, they are undetectable by traditional screening methods or vapor must be produced from these materials. Consequently, using current technology, sample chemicals must be first collected from a surface by wiping or vacuuming. The wipes or vacuum filters must then be heated and the vapor introduced to a vapor detector for detection and identification of the chemicals present. These methods are time consuming, expensive and highly dependent upon trained operators capable of near perfect consistency in obtaining samples. These factors limit screening to only a small portion of the samples that should be examined.
The present invention provides a complete means to scan surfaces such as paper, plastics, skin, glass and textiles from variable distances and determine in seconds if targeted chemicals or materials are present, completely independent of the vapor pressures of such chemicals. Currently available detectors generally create ions of the vapors of targeted chemicals and other chemicals taken into the body of the detector, then separate the ions and detect, identify and provide notification of the presence of any targeted chemicals. The present invention overcomes this limitation by creating ions from sample chemicals exterior to the detector, on surfaces, and draws these ions into the detector for separation, detection, identification and notification.
In order to do this in an easy to use, yet economical configuration, reactant ions are created within the detector from a constant supply of conditioned air or other gas. These reactant ions are focused and accelerated as they leave the detector. The reactant ion stream impacts the chemicals on a surface exterior to the detector and creates surface sample chemical ions. These ions are drawn into another part of the detector, using electronic means to control ion movement and collection. Once within the detector, the surface sample chemical ions are separated from the ambient air in which they are collected, and simultaneously moved and concentrated in a stream of constant composition air. The ions are then detected and identified after movement into a micro differential mobility spectrometer having no moving parts and made much like an integrated circuit.
In seeking to develop a single device that would directly ionize samples on surfaces and subsequently detect and identify these sample ions, use was made of several precedents. For example, ion mobility spectrometers require an ion source, which may be a radioactive ionization source (β-emitter or electron producer) such as 63Ni. Because of the regulations associated with obtaining, transporting and maintaining equipment with radioactive sources, alternative ionization sources such as corona discharge ionization sources are preferred. Such a corona discharge is described in U.S. Pat. No. 6,225,623. Corona discharge units have been and are widely used with helium gas to produce long-lived metastable helium atoms. These excited state helium atoms are used to transfer energy to neutral molecules, thereby ionizing such neutral molecules. An exemplary detector that uses a corona discharge ionization source is described in the Cook patent, U.S. Pat. No. 4,789,783. Such corona discharge ionization detectors are commercially available from Finnegan, GOW-MAC, VICI and others.
A variation of the corona-type ionization source is described in International Application No. WO 2004/098743 A2. The source comprises a chamber having an inlet port and an outlet port for passage of a carrier gas, and a pair of electrodes arranged to create a corona discharge within the chamber. The carrier gas, helium or nitrogen, is passed through the corona discharge causing formation of, among other species, neutral, excited state, metastable species of the carrier gas. Those excited state carrier gas molecules upon leaving the device then contact the sample, or analyte, and by transfer of energy from the excited state carrier gas molecules to analyte molecules, produce analyte ions. The analyte ions in the carrier gas are then passed to a charged particle or ion sensor, which may be the sensing element of a mass spectrometer or an ion mobility spectrometer. In this teaching, the helium metastable atoms leave the confines of the device and subsequently react with surface materials to produce surface sample ions.
In the present invention, the neutral helium metastable and energetic atoms, freed of any ions produced in the corona discharge by ion filters, are then reacted with the chemical components of air or other gases such as dopants, introduced within the device, causing the formation of reactant ions, such as O2− and H3O+, and electrons within the device. It should be noted that neutral metastable helium atoms can interact with other metastable atoms and with neutral ground state atoms to produce charged species. So, even if ion filters are used at this stage, ions can still be produced. The advantage of using filters is that they remove the ions produced during passage of the gas through the corona discharge. Subsequent charged species production results only from the interactions noted above. Alternatively, if ion filters are not used, ions produced in the corona discharge, along with energetic neutral species, are reacted with the chemical components of air or other gases, introduced within the device, causing the formation of reactant ions, such as O2− and H3O+, and electrons within the device. This transfer and downhill flow of energy from (1) the corona to energetic atoms and ions, then (2) from energetic atoms and/or ions to ions of introduced gases, results in the production of reactant ions that can be controlled and used in “soft” ionization processes to produce significant populations of molecular ions and clusters from a very wide variety of chemicals.
Unlike helium metastable atoms, which carry no charge, these reactant ions are positively or negatively charged and can be focused and accelerated within the device, and after they leave the device, they can be moved towards or away from different parts of the device depending upon the potential applied to that part of the device. Furthermore, these reactant ions can react with most chemicals to produce ions from those chemicals. The electrons produced can also react with gases to produce reactant ions.
The capability to control the reactant ions through focusing and acceleration provides several useful practical advantages over other methods. First, automated distance information from a rangefinder can provide feedback control to the ion focusing and accelerating portions of the invention. For example, by using such feedback control to direct the electronic focusing to vary the width of the ion stream leaving the ion production device, the reactant ions created within the invention can be focused on a surface such that the same amount of ions per unit area hit the surface independent of the distance between the detector and the surface. This allows the operator freedom of movement of the detector away from and towards the surface with assurance that, regardless of position, the same amount of detector initiated reactant ions will generate the same amount of sample ions on the surface, and ultimately, the same sensor-driven signal within the detector. Second, the reactant ions created within the invention are those well known to react with a wide variety of chemicals of interest, to form predominantly molecular ions. Molecular fragmentation is kept to a minimum in this “soft” ionization process. This greatly simplifies the detection and identification process. Third, the reactant ions emitted from the detector can be confined within a sheath gas such that, in the transit between detector and surface, the integrity of the detector originated ion population is largely maintained and admixing with the ambient air between the detector and the surface sample is kept to a minimum.
Turning to the detection of the produced surface sample ions, a mass spectrometer would appear to be ideal because of its high sensitivity and resolution or selectivity. However, mass spectrometry requires large, heavy and expensive equipment making the technique impractical for applications that require portability. The most widely used analytical systems for detecting and monitoring explosives and chemical warfare agents, both by the military and for airport security, employ ion mobility spectrometry (IMS). Ion mobility spectrometers function by pulling a gas that contains molecules of the compounds of interest through an ionization source and then moving the ions produced through a sensor. Both the ionization source and the sensor are commonly incorporated within a cylindrical drift tube, which is divided into two parts. The first, or reaction, region contains the ionization source and is separated from the drift region by an electrical shutter or ion gate. In all cases, the sample molecules are directly subjected to the ionization source and, depending upon the sample and the intensity of the source, a wide variety of molecular fragments, as well as simple ions, are produced. Under the influence of an electric field, the mixture of reactant and product ions reaches an ion gate that separates the reaction region and the drift region. With a bias voltage applied, the ions are attracted to the ion gate and lose their charge. Then the bias is briefly turned off, and ions are transmitted into the drift region of the cell. The smaller, more compact ions have a higher mobility in the electrical field than the heavier ions, and therefore traverse the region and collide with the collector plate in a shorter time. The collector current is then amplified. Its magnitude, as a function of time, is proportional to the number of ions arriving at that moment. The time-of-flight or mobility enables the identification of different chemicals. There are several significant drawbacks to IMS including:                Typical ion mobility spectrometer analysis cycles require 5–8 seconds from introduction of sample to alarm notification        The percentage of ions produced that are actually detected is as low as 1% due to the ion gate, resulting in lower sensitivity        Resolution among different ions is dependent upon the length of the drift region, making it difficult to miniaturize        Reduction in the cross sectional area of the drift tube also decreases sensitivity, again making it difficult to miniaturizeDespite those limitations, ion mobility spectrometers may be usefully employed with the ion source of this invention to produce portable, non-contact sampling systems.        
Another charged particle or ion sensor that is coming into use employs differential mobility spectrometry (DMS). An example of a differential mass spectrometer is the MicroDMx manufactured by Sionex Corporation. This device has no moving parts and is microfabricated. Its small size allows for extremely fast clear down times and very rapid responses to the presence of ions. In differential mobility spectrometry, selectivity is significantly enhanced relative to other techniques of ion resolution and detection. DMS exploits the way in which the mobility of ions changes in response to changes in an applied variable high electric field, and this provides substantially more information relating to a molecule's identity than other methods, consequently leading to a significant reduction in false positives. Differential mobility spectrometry can detect positive and negative ions simultaneously and has superior sensitivity and selectivity capabilities relative to more commonly used sensors such as ion mobility spectrometers. DMS achieves superior selectivity relative to simple time-of-flight information employed in other detectors by using placement of ions within four-dimensional space constructed to examine changes in ion mobility as a function of changes in high electric field strength. Detection and identification are rapidly made and notification of presence or absence of targeted materials given in near-real time. Sensitivity is enhanced as well because as a range of compensation voltages in a DMS device are scanned the actual percentage of ions detected for any type of ion species is significantly higher (>10×) than in conventional IMS. The capability of DMS to continuously accept and analyze sample ions, without the need for the ion-gate used in IMS devices, also increases the percentage of ions detected and consequently increases its overall sensitivity. Therefore, the sensitivity of DMS is higher than that of conventional IMS, and DMS sensors have the capability to detect compounds in the parts per trillion ranges. Differential mobility spectrometry can be used to detect positive and negative ions simultaneously. This is important in cases where all surface sample ions created would be collected at the same time, or where positive and negative ions would be alternately collected for extremely short times. These attributes of DMS are very important for the detection of explosives or other dangerous or controlled materials on clothing, baggage, paper, etc. at security checkpoints. Detection of such materials must be rapid, but also must be done with virtually no false negatives such that these materials go undetected when actually present, creating a potentially dangerous situation. There must also be virtually no false positives such that materials are detected when none are present, thereby closing down the checkpoint while the false positive is verified as erroneous. The selectivity of DMS for certain materials such as explosives can be enhanced by transferring ions from an incoming ambient air stream to an air stream of controlled composition, possibly containing a dopant chemical to further control the nature of the ion species in the stream.
Having considered the ion production and ion detection portions of the invention, it is then necessary to manage, in a complementary manner, the movement of the reactant ions from the detector to the surface and the subsequent collection and concentration of surface sample ions in another part of the detector in order to most efficiently use the ions produced within the invention and in order to maximize sensitivity of the invention. Issuing reactant ions of alternating charges as a function of time from the ion production device and biasing the ion outlet to the same charge of the reactant ions so the ions are “pushed” away from the ion outlet and towards the surface can accomplish this. In synchrony with the changing biasing of the ion production device, the ion collection device undergoes programmed biasing aimed at providing sufficient charge opposite to that of the produced surface sample ions, thereby “pulling” these ions toward the collection device and into the sensor for detection and identification. The maximum possible number of collected ions must reach the sensor to attain the highest sensitivity. In order not to lose ions through collisions with walls within the detector, the ions are focused such that they are transported without touching the walls. The possibility exists that reactant ions of one charge could form both positively and negatively charged surface sample ions. In this case, for each “burst” of reactant ions released on the surface sample, there would be two cycles of ion collection—one positive and one negative. This allows for the real-time collection of maximum information from the surface sample. The continuous detection of residual or unreacted reactant ions by the differential mobility spectrometer provides a means for feedback and other control of the detection system. For example, such feedback can be used, in conjunction with the distance from the detector reactant ion production device to the surface (provided by a rangefinder) to control the timing of changes of potential applied to the ion collection inlet, relative to those changes of potential controlling the production of reactant ions, as the distance between the detector and surface is changed. This has the practical effect of providing assurance that relatively the same number of ions is detected by the detector as it is moved toward or away from the surface. The operator, therefore, does not have to keep the detector at a fixed distance from the targeted surface and allows for freedom of movement of the detector toward or away from the surface with assurance that targeted surface materials will still be detected with relatively the same certainty. In the absence of a surface, i.e. if the targeted chemical is contained in the ambient air, feedback control without using the rangefinder but using the DMS signal, can be used to control the density of reactant ions projected from the ion production means, thereby controlling the overall sensitivity of the detector.
Using a means to generate ions of targeted chemicals on surfaces coupled with a small fast sensor with excellent sensitivity and selectivity, and the means to use distance and sensor information as feedback to control the entire process, provides the elements of a detector that can be used to close security loopholes. It will enable the rapid screening of the surfaces of people, baggage, cargo, parcels and vehicles at government and private facilities, transportation centers, checkpoints and borders, among others. It will also find use in substantiating illegal activities by facilitating the rapid and accurate detection of chemical warfare agents (CWAs), explosives and illicit substances and to verify decontamination efforts are successful by military personnel. Key features of the invention are means to control, focus and accelerate the detector originated reactant ions responsible for producing surface sample ions from chemicals on surfaces, and the coordination of these events with the rapid collection of the surface originated ions in high yields for detection and identification by the sensor. The capability to apply roughly the same amount of reactant ions to the same surface area regardless of the distance of the detector from the surface allows the operator to scan the surface from variable detector—surface distances and obtain the same result, rather than be constrained to holding the detector at a fixed, close distance from the surface.
Hence, it is an object of this invention to provide an ion production and sensor system that operates by impacting a reactant ion stream upon a surface to form ions of sample compounds carried on that surface, to collect at least some of the sample ions that are formed, and to pass those ions into, for example, a differential mobility spectrometer to identify and quantify the sample compounds.
Another object of this invention is to provide an extremely sensitive, fully portable, hand-held detector that can identify and quantify compounds such as drugs and chemical warfare agents in place on surfaces without physical contact of those surfaces.
Yet another object of this invention is to detect equally well the presence of sample compounds having extremely low or hugely different vapor pressures without physical contact of the surface that carries the sample compounds.
It is a further object of this invention to provide an improved reactant ion production means that can direct a beam of reactant ions upon a surface to produce sample ions from materials on the surface at atmospheric pressure and without physical contact.
Other objects and advantages of this invention will be evident from the following description of certain preferred embodiments.